Hybrid drives for motor vehicles are being increasingly developed at this time, in which the hybrid drive is implemented in that, in addition to the internal combustion engine of the vehicle, a further drive, such as an electrical drive, is built in. The usefulness of this development lies mainly in the reduction of fuel usage and the low emissions of such drives. In hybrid drives, hydraulic drives are also used as additional drives, besides electrical machines. The use of hydraulic drives extends in a heavily concentrated way to construction equipment, fork lift trucks and military applications.
Besides combustion engines having externally supplied ignition, drives may also include self-igniting internal combustion engines, such as a Diesel engine. The injection of fuel into the combustion chambers of self-igniting internal combustion engines, such as Diesel engines, is made via high pressure injection systems such as high-pressure reservoir injection systems (common-rail). These fuel injection systems consequently represent a component of hybrid power trains. Vehicles that include a hybrid drive, which as an internal combustion engine includes a Diesel engine, may get into driving situations, such as purely electrical operation, start-stop situations or recuperation operation in which the actual pressure (actual system pressure) prevailing in the high-pressure reservoir (common-rail) does not agree with the setpoint system pressure of the respective driving situation of a conventional drive, that is, a power train having only an internal combustion engine, in this case a Diesel engine.
In the state of development reached today, with respect to high-pressure reservoir injection systems, such as a common-rail injection system, pressure-maintaining functions are already being used in internal combustion engine stop-start functions. For this purpose, immediately before shutting down the internal combustion engine in vehicle stopping phases, the fuel pressure in the high-pressure reservoir (common-rail) is not reduced, but is kept in reserve using a pressure-maintaining valve, so that when there is a restart of the internal combustion engine, the required fuel pressure-injection pressure in the high-pressure part of the high-pressure reservoir injection system is directly, or rather very rapidly available. Because of the pressure-maintaining valve, which is associated with the high-pressure reservoir, the pressure prevailing in the high-pressure reservoir is able to be maintained in the ideal case, and consequently the system pressure is able to be maintained. If a creeping pressure loss takes place, this will be very slight, so that the required system pressure will be able to be built up again very rapidly by the high-pressure pump applied to the high-pressure reservoir (common-rail). Very short starting times of the internal combustion engine are necessary for the acceptance of stop-start systems on the part of the user in the vehicle.
While a method for controlling a vehicle drive unit is discussed in German patent document DE 10 2005 040 783 A1, in which a continual comparison is made between an overall actual torque and an admissible overall torque, a system pressure monitoring in high-pressure reservoirs using a pressure-maintaining valve is already known from the related art, to be sure, but a situation-dependent pressure adjustment, as may occur within the scope of hybrid drives, such as during the transition from purely electrical driving or from recuperation operation to normal internal combustion engine operation, is not available yet.
German patent document DE 10 20005 040 783 A1 relates to the continual comparison of an overall actual torque to an admissible overall torque, the overall actual torque being calculated from individual actual torques of at least two individual engines and the admissible overall torque being calculated from admissible individual torque values of the at least two individual engines, and then a fault reaction being initiated if the comparison reveals that the overall actual torque is greater than the admissible overall torque. These design approaches known from the related art relates only to an admissible overall torque. Whereas in stop-start systems in conventional vehicle drives, the restart of the internal combustion engine, be it a self-igniting internal combustion engine or one having externally supplied ignition, takes place in driving situations in which no load, or rather a small load has to be raised, in vehicles equipped with hybrid drives, a restart of the internal combustion engine is executed also at high loads, and thus also higher fuel setpoint pressures in the fuel accumulator injection system (common-rail).